Add validation for Kitfiles

[amisevsk]

Description

Add some basic validation to Kitfiles, printing an error if

• Multiple layers share the same path (makes unpacking/ignoring unclear)
• Any layer is an absolute path rather than relative
• Model part's type field is invalid: (max 64 characters, alphanumeric with ., -, and _; must start with alphanumeric)

Testing

You can see all of the failure cases by trying to pack the following Kitfile

Kitfile

```yaml manifestVersion: 1.0.0 package: name: test-kf model: name: my-model path: path1 parts: - name: part1 path: path1 - name: part2 path: /abs/path - name: part3 path: valid-path type: test type with space - name: part4 path: also-valid type: really-long-but-otherwise-valid-type---this-line-is-65-chars-long datasets: - name: dataset1 path: path1 - name: dataset2 path: path3 - name: dataset3 path: /abs/path/2 code: - path: path1 - path: path3 - path: /abs/path/3 ```

Linked issues

Closes #81